I am positive that really thick and long manes are genetic. You can get a horse who is not genetically predisposed to grow a fairly good lengthed mane, but it won't have the thickness of a horse who is meant to have great hair.
We braid manes and redo them once per week. We take care not to get the mane braids wet when the horse is washed and never braid a horse who is going to be out in the rain. Also, never braid a pasture horse or you will have more hair on your trees and in your fences than on the horse. Once weekly, undo the braids, rinse the mane, and apply a hot oil treatment to it. Let the hot oil treatment stay on for at least 10 minutes up to 30 minutes, then rinse. Once it is COMPLETELY DRY you can rebraid it. Damp getting into your manes can cause them to mold and fall out. If you'd like, you can put in some leave in conditioner before you braid, but it does tend to make the mane hair more slick and difficult to braid.
Be prepared, if you do braid your horse, to see some hair loss when you undo the braids and at the bed of the mane. This is normal hair loss but the hair is held captive in the braid and will not just slough off the horse. Never, never, never use a brush on the manes. Finger combing only. Avoid shampooing any more frequently than once monthly and do not use alcohol based sprays (Show Sheen, etc.) on the mane at any time. If you must use Show Sheen for your show grooming routine, make sure the horse is washed well with clarifying shapoo at the end of the show to get that drying alcohol out of the haricoat and mane/tail hairs. Personally, I don't use Show Sheen or any like product due to the alcohol content, but everyone has a different way they like to turn out their show horses. That's about it.